Halftoning is a process that is used to convey gray scale information in printers which typically can print only black or white. Many halftone concepts and terms now used in electronic printing originated with the classic offset printing press. Printing presses can usually print areas of single intensity as they have only an ability to apply ink to a page or not apply ink to the page. This limited ability results in only two colors, i.e., that of the ink and that of the print media. By varying the size of printed dots, however, it is possible to give the impression of various shades of gray.
In electronic black and white printers, gray scales are accomplished by building a palette of grays that consists of clusters of black dots. A given cluster with more black dots is darker, while a cluster with less black dots is perceived as a lighter gray.
Halftone principles and procedures are applicable to color printers as well. In a color printer, the halftone technique is applied to each color plane (usually Cyan, Magenta, Yellow and blacK (CMYK)). Instead of generating only shades of gray, the printer provides mixtures of varying intensities of the four color planes. Layering of those variable intensity color planes enables the printing of a full color document.
Current implementations of halftoning techniques in laser printers are best suited to one type of a printable object (such as an image, text, line art, etc.). Many times when a printer employs a particular halftone technique with respect to a combination of printable objects, one printable object looks bad while another printable object looks good. Borders between printable objects can also show visually unappealing artifacts. In most laser printers, halftone patterns used by the printer to achieve a halftone effect are fixed within the printer's read-only memory. Thus, to correct a visual artifact or poor print quality, the user must change the printable object to be printed to accommodate the halftone pattern.
In prior solutions to the halftoning problem, design engineers chose a halftone pattern or an algorithm that was a reasonable compromise between various printable objects. A halftone was chosen to minimize the artifacts between printable objects, while at the same time providing a reasonable approximation of a continuous tone in the print output. The disadvantage of this approach was that the print quality for some printable objects was sacrificed by the compromise. As an example, if text was included within a picture image, and a single halftone technique was applied to the combined image, a visually unappealing border often occurred between the text and the image. More specifically, the crisp edges associated with the text were lost or, in the alternative, the picture showed objectionable contour lines where there should have been gentle transitions of gray scale.
Currently, a host processor operating under the Windows operating system (a Trademark of the Microsoft Corporation) employs a functionality within the Windows operating system called "graphics device interface" to perform the assembly and transmission of a color image to a printer. The color data is transmitted to the printer as a data stream of 3 or 4 eight bit color values per pixel. The host processor transmits to the printer three successive image planes, each plane comprising a single color of the color image data (i.e., red, green and blue or RGB). Thereafter, the printer subjects the individual color plane image data to a rasterization procedure so as to achieve an intermediate page format which, while not yet printable, is in raster form.
In the prior art, halftoning was performed during the rasterizing operation. Because raster operations modify the image data, constraints were placed on when halftoning could be performed and what kinds of halftoning could be applied. Further, the prior art halftoning procedure subjected all image data on the page to a common halftoning procedure, with a result being that the halftoning action was a compromise between desired resolution and desired intensity. Lastly, if, during an overlap operation, images having different line per inch resolutions are superimposed, the exclusive OR procedure described above causes undersized interference patterns in the image.
Accordingly, it is an object of this invention to provide an improved method and apparatus for halftoning of images which enables plural halftone procedures to be applied to a single image.
It is another object of this invention to provide an improved method and apparatus for halftoning of images wherein the halftone operation is applied to color planes comprising an already-rasterized image.
It is yet another object of this invention to provide an improved method and apparatus for halftoning of images wherein tables that are employed during the halftone operation can be altered so as to achieve improved halftone results.